In an electrophotography method, an electrostatic image is formed on a photosensitive body by an exposure step, and the electrostatic latent image is visualized by developing with a developer containing a toner, followed by subjecting a transfer step and a fixing step.
The toner is generally produced by a kneading and pulverization method. Inorganic or organic fine particles are added to the surface of the toner produced by this method depending on necessity, and a toner exhibiting excellent performance can be produced by this method. However, it involves the following problems.
In the general kneading and pulverization method, a toner may form fine powder or may suffer change in its shape due to a mechanical shearing force applied to the toner in a developing apparatus. It brings about problems in that the fine powder sticks on a surface of a carrier to accelerate charge deterioration of the developer, the particle size distribution is broadened to cause scattering of the toner, and the development property is deteriorated by the change of the shape of the toner to cause deterioration of the image quality.
In the case where a releasing agent such as a wax is internally added to a toner produced by the pulverization method, the wax present on the surface of the toner is easily transferred to surfaces of a development roll, a photosensitive body and a carrier by a mechanical force to contaminate them, and thus the reliability is lowered.
Furthermore, because the shape of the toner is irregular, the fluidability of the toner is deteriorated with the lapse of time, and a fluidizing agent is buried in the interior of the toner to deteriorate the developing property, the transferring property and the cleaning property. When the toner recovered in the cleaning step is reused in the developing apparatus, the image quality is further deteriorated. In the case where the amount of the fluidizing agent is increased to prevent these problems, another problem occurs in that black spots are formed on the surface of the photosensitive body, or the particles of the fluidizing agent is scattered.
In recent years, as a method for positively controlling the shape and the surface structure of the toner, a process for producing a toner by an emulsion polymerization aggregation method has been proposed in JP-A-63-282752 and JP-A-6-250439. However, it still involves problems in stabilization of the peelability of a fixing sheet on fixing and the transparency on printing on an OHP sheet.
In a recent digital full-color duplicating machine and printer, the demand of which is being increased, development is conducted by utilizing subtractive color mixing using developers of Y (yellow), M (magenta), C (cyan) and Bk (black), and therefore an image is formed by using a larger amount of the developer than the machine for forming a monochrome image. Furthermore, in addition to a text image, which has been conventionally, printed, duplication or printing of a solid image, such as a photograph or a picture, is frequently conducted in such an apparatus. In view of such circumstances, high reliability on fixing in a low temperature range is demanded.
In order to satisfy the demand, it is required to obtain an image of good quality by certainly adhering a toner image to paper by high-speed fixing, i.e., application of heat and pressure in a short period of time, without forming offset in a fixing roll and also without causing damage in the image due to stress after fixing.
As a measure for satisfying the demand, for example, JP-A-8-101531 proposes a toner, in which an extremely low molecular weight component is excluded from the binder resin, but unevenness of gloss and damages due to bending are liable to occur.
With respect to the peelability of an image from a fixing roll, i.e., so-called releasability, a method is frequently employed in that a releasing agent is uniformly coated on the surface of the fixing roll particularly in a color-duplicating machine (JP-A-4-308878). However, there are problems in that the effect of the releasing agent is largely lowered due to deterioration of the releasing substance with the lapse of time, the cost is increased due to the fixing device becoming large and complex, and the releasing substance is transferred to the surface of the receiving material so that writing with a ballpoint pen on the receiving material becomes impossible, and an adhesive tape cannot be attached to the receiving material.
In order to avoid the problems, JP-A-5-61239 proposes a toner for oil-less fixing containing a large amount of releasing substance, but the releasability cannot be stable. The Dispersibility of the materials within the toner largely influences not only on the adhesion of the fixed image on paper, the releasability of the fixed image from the fixing roll, and the bending resistance and the gloss after fixing, but also on the total fixing performance such as the transparency on an OHP sheet.
As a method for improving the Dispersibility of the releasing agent, JP-A-2-105163, for example, proposes to introduce a resin having a polar group to improve the encompassment and oozing of the releasing agent, but it cannot sufficiently improve the fixing property.
As another method for improving the dispersibility of the releasing agent, JP-A-4-188156 proposes to previously treat the surface of a coloring agent, but it involves the problems described above, and also the stable transparency on an OHP sheet is difficult to be obtained.
As described in the foregoing, the behavior in molten state and the control of structure formation of the toner and the constitutional components thereof are important for the adhesion to paper and the peelability from the fixing roll of a toner image, and the Dispersibility of the releasing agent and the coloring agent. The quantitative determination of those properties is generally achieved by using, as the standard, the relaxation modulus of elasticity and the relaxation time obtained from the measurement of dynamic viscoelasticity.
In general, in the case where a distortion is applied to an article to be measured, such as a toner, the stress thus generated exhibits exponential decay behavior, in which the stress S after the lapse of time t.sub.1 is expressed by S=S.sub.0.e.sup.-1/t, where S.sub.0 is the initial stress, and the time where t.sub.1 agrees to t is determined as the relaxation time. The relaxation modulus of elasticity is a value obtained by dividing the stress S by the deformation amount.
The stress-relaxation behavior is greatly influenced by the viscoelasticity of the binder resin and the structure, the size and the amount of the releasing agent dispersed in the resin, and the molten state thereof can be expressed by the relaxation behavior, i.e., the relaxation modulus of elasticity and the relaxation time. However, there has been no example of positively applying them to the molten behavior and the viscoelasticity control on fixing of the toner.